The major objectives of this proposal are to establish: (1) the extent of modification of the NH2-terminal region of newly initiated eukaryotic and prokaryotic proteins; (2) the basis for the modification reactions; (3) the relative frequency of occurrence of these reactions; and (4) the stage in protein maturation at which modification occurs. In addition, a specific mechanism for control of protein synthesis at the translational level will be examined. These objectives will be pursued by structural studies on the NH2-terminal region of eucaryotic and procaryotic proteins initiated in vitro and in vivo. Two types of modification, one resulting from the hydrolysis of the initiating methionine and the second resulting from reactions leading to blocked alpha-amino groups, will be examined. These studies will be carried out using Escherichia coli, mouse liver, Ehrlich ascites cells, and mouse L- cells. Realization of these objectives should: (1) provide information bearing on the mechanisms and significance of the modification of the NH2-terminal region of newly initiated proteins; (2) indicate any significant differences in these reactions as they occur in normal or tumor cells; and (3) provide information useful for testing models for control of protein synthesis at the translational level.